These small round guys are actually encrusting brachiopods. I found similar pictures in Tunnicliffe and Wilson (1988) calling them Crania californica, but from WorMS it looks like that name has been updated to Novocrania philippinensis.
Tunnicliffe and Wilson (1988) Brachiopod populations: distribution in fjords of British Columbia (Canada) and tolerance of low oxygen concentrations. Mar. Ecol. Prog. Ser.
http://www.marinespecies.org/aphia.php?p=taxdetails&id=235392